1. Field of the Invention
This invention is provided for the analysis of the wavefront of a light beam.
Such a type of analysis makes it possible to test optical elements, to qualify optical devices, as well as to steer deformable optical components as used in active or adaptive optics. It also allows for the study of non directly measurable physical phenomena, such as variations of optical index within turbulent media that can be encountered when crossing the terrestrial atmosphere, as well as in a blower vein. It is also used for testing the planarity of electronic components, for example matrix focal planes, as well as for shaping power laser beams.
2. Description of Related Art
The type of analysis of a wavefront according to the invention is based on the use of a diffraction grating positioned on the path of the beam to be analyzed.
For a better understanding of the following, such a grating is defined as being an optical system introducing periodic variations of phase, intensity or phase and intensity. Any diffraction grating is thus characterized by the multiplication of two functions: the one, referred to as phase function, represents the periodical variations of phase introduced by the grating and the other one, referred to as intensity function, represents the periodical variations of intensity introduced by the grating.
The French patent application No. 2,712,978 (which corresponds with U.S. Pat. No. 5,606,417) discloses the mode of construction and definition of a two-dimensional grating. A set of points regularly arranged, according to two directions constitutes a planar meshing. Such points define an elementary mesh. The elementary mesh is the smallest surface allowing for a non-lacunary paving of the plane to be reached according to both directions defining the latter. The polygon of the elementary mesh is the minimal surface polygon having the sides thereof supported by the mediatrices of the segments connecting any point of the set with its nearest neighbors. A two-dimensional grating is the free repetition of an elementary pattern arranged according to a planar meshing. A planar meshing can define elementary meshes, being either hexagonal or rectangular (square meshes being only a special case for the latter).
When a diffraction grating is being illuminated with a light beam, referred to as an incident beam, light beams being diffracted by the grating, also called emerging beams, can be described using two equivalent approaches.
The first approach consists in considering emerging beams as replicas of the incident beam. These are called sub-beams, each corresponding to one diffraction order of the grating. Amongst such sub-beams, two categories are to be distinguished. First of all, there are classified sub-beams, referred to as main beams corresponding to the diffracted orders being used according to the invention. The other orders, being not useful for analysis, are called secondary sub-beams. The grating will be thus defined so as to favour the emergence of the main sub-beams and to minimize the presence of the secondary sub-beams.
The second approach consists in considering emerging beams as beams being diffracted by each mesh of the grating. These are called secondary beams.
When an intensity function is introduced by a grating, each secondary beam results from a mesh of the intensity grating called sub-pupil.
In the aforementioned French patent application No. 2,712,978, a three-wave lateral shearing interferometer implements a two-dimensional phase and/or intensity grating and a spatial filtering system. According to the approach through decomposition into sub-beams, the grating optically subdivides the incident beam to be analyzed into a plurality of sub-beams in a conjugated plane of the defect. A system for spatial filtering the sub-beams is intended to select three main sub-beams used for analysis. A particular optical processing of the three sub-beams obtained in this way makes it possible to observe an interferogram made of a hexagonal meshing of light spots having a contrast being invariant, whatever the selected observation plane is. This interferogram is sensitive to gradients of the wavefront and this with a continuous adjustment possibility for dynamics and sensitivity. The observation distance is there defined as the distance separating the selected observation plane from the so-called zero sensitivity plane, this latter being a conjugated plane conjugated with the plane of the grating located downstream the spatial filtering. Such a type of interferometer has the advantage of displaying important metrological qualities because of the frequency purity of the generated interferogram. Moreover, the measurement error can be estimated from the measurement itself. Finally, such an interferometer can operate with polychromatic light, provided the path difference of the defect to be detected does not depend on the wavelength.
On the other hand, it is complex to be implemented, because of the insertion of the spatial filtering system for selecting the main sub-beams between the grating and the observation plane of the interference fringe system. Moreover, the spatial filtering system brings limitations for measuring strongly disturbed light beams or light beams with a very large spectrum width.
The French patent application No. 2,795,175 (which corresponds to U.S. Pat. No. 6,577,403) discloses an interferometer of a four-wave lateral shear type being a development of the interferometer with a three-wave lateral shearing as being described hereinabove. The grating at the basis of such an interferometer optically subdivides, in a conjugated plane of the defect, the incident beam to be analyzed into four main sub-beams, useful for analysis. As secondary sub-beams are minor with a low amplitude, removing them by a spatial filtering system is not necessary. The interferogram comprises a rectangular meshing of light spots, the contrast of which is invariant, whatever the selected observation plane is. Like the interferometer of a three-wave lateral shearing type, such an interferometer can operate with polychromatic light and offers a continuously set sensitivity and dynamics by a simple translation of the observation plane with respect to the so-called zero sensitivity plane. Moreover, as opposed to the three-wave lateral shearing interferometer, the absence of a spatial filtering system offers a better implementing ease and makes it possible to measure strongly disturbed light beams or light beams with a very large spectrum width. Estimating the error is also possible by means of such an interferometer, however, it will be less robust in the case of the measurement of high dynamics defect. In addition, the sampling geometry of wavefronts to be analyzed in the four-wave lateral shearing type interferometer is less optimal than that achieved with a three-wave lateral shearing type interferometer.
Thus, it seems highly desirable to provide an interferometer combining, on the one hand, the implementing simplicity and the operating capacity, from highly disturbed low intensity light sources or light sources having a very large spectrum width of the four-wave lateral shearing type interferometer and, on the other hand, the possibility to estimate the measurement error robustly and the optimum sampling geometry of the wavefronts to be analyzed of the three-wave lateral shearing type interferometer.